1. Field of the Invention
The invention relates to what is referred to herein as xe2x80x9cpatternation,xe2x80x9d performing quantitative measurements of the specific properties of particles, e.g., surface area and/or mass distributions, within a particle field such as a spray, including dense particle fields. More particularly, the invention relates to an optical technique for determining such distributions.
2. Description of the Related Art
Particle fields comprising collections of particles, such as liquid particles or droplets in sprays, are encountered in many aspects of daily life. Sprays and other particle fields are associated with water sprinklers, paint sprayers, chemical process plants, medicinal coatings for tablets and pills, aerosols for inhalation therapy, lubrication, cooling, fabrication techniques using molten metals, and fuel and/or oxidizer injectors for heating and power generation, diesel and spark ignited reciprocating engines (e.g., automotive applications), gas turbine engines for air, marine, and ground applications, and rocket engines. The efficacy of the sprays in most applications depends largely on the spatial distribution of the particles, i.e., droplets of liquid, or its xe2x80x9cpattern,xe2x80x9d produced by an atomizer. Consequently, a method that would quickly and reliably characterize this pattern has been anxiously sought for many years. Such a method could also be used for quality control in the production of atomizers.
Optical techniques for determining distributions in particle fields are attractive since they are non-invasive and generally offer relatively high spatial and temporal resolution. Furthermore, they are robust in harsh environments and can be applied in any physical orientation. An optical patternator generally involves the use of laser light which is directed into the particle field being investigated. In one optical patternator technique, laser light is tuned to an absorption peak in molecules which are either inherent in the field or have been added as dopants. The molecules fluoresce at a wavelength other than that of the incident light in a process known as laser induced fluorescence (LIF). The LIF is detected by an optical detector, which is usually positioned some distance from the sample. The amount of light emitted from a given volume in the particle field is proportional to amount of light incident upon that volume, and the total number of emitting molecules in the volume. The total number of emitting molecules will in turn be proportional to the total amount of mass in the volume, regardless of whether this mass is contained in one particle or whether the mass is distributed over several particles in the volume. An xe2x80x9camount of lightxe2x80x9d is defined herein to mean the total light energy crossing a surface over a given period of time. For a continuous light source, the given period of time may be more or less arbitrarily chosen. For a pulsed light source, the given period of time is the duration of the pulse. Similarly, an xe2x80x9camount of signalxe2x80x9d or xe2x80x9csignal amountxe2x80x9d is defined herein to mean the total signal energy crossing a surface over a given period of time. The terms xe2x80x9cscattered signalxe2x80x9d or xe2x80x9cLIF signalxe2x80x9d may sometimes be used herein to distinguish the scattered light and the LIF light from the original excitation light.
In another optical patternator technique, laser light scattered by particles in the field is collected. This scattered light is not shifted in wavelength, and the amount of scattered light is proportional to the amount of incident laser light. Under certain conditions, particularly when the particles are spherical and the wavelength of the light is much smaller than the size of the particles, the amount of scattered light can also be proportional to the total surface area of the particles contributing to the scattering.
Optical patternation techniques, however, generally suffer from shortcomings related to the scattering of light. For example, whereas the amount of scattered light can sometimes be theoretically related to the total surface area in a volume of the particle field, and whereas the amount of LIF emitted from a volume in the particle field is theoretically related to the amount of mass in the volume, the amount of detected scattering and/or LIF signal is not in general proportional to the area or mass distribution for at least the following reasons. First, the particle field to be investigated will scatter and/or absorb some of the incident laser light, e.g., at the surfaces of the particles. If enough particles are present, the amount of the exciting laser light can be considerably reduced, resulting in an under-representation of the amount of material that is actually present. This effect is referred to herein as xe2x80x9cextinctionxe2x80x9d of the incident laser beam. Secondly, some of the emitted LIF and/or scattered light will be reabsorbed and/or scattered by particles within the field before reaching the optical detector, leading to a further under-representation of the amount of material that is actually present. This phenomenon is referred to herein as xe2x80x9csignal attenuation.xe2x80x9d Third, light scattered by particles illuminated by the laser strike other particles not directly illuminated by the laser. These other particles also scatter and/or fluoresce, adding an unknown amount of additional signal from locations other than at the measurement point, in an effect referred to herein as xe2x80x9csecondary emission.xe2x80x9d
Thus, there is an need for improved methods of accurately determining property-specific distribution such as the surface area and/or mass distributions of particle fields.
In one aspect of the invention, to measure property-specific distributions like surface area and mass concentrations in a particle field that attenuates light passing therethrough, such as a spray or aerosol, a probe beam having a suitably selected wavelength is directed into and thereby caused to interact with the particle field. Light produced by this interaction is detected by sensors separated a distance from the beam. Extinction of the probe as it propagates through the particle field is quantified by determining the amount of light exiting the field in comparison with the amount of light entering the field. The property specific concentrations are determined from the light detected by the sensors after correcting for the extinction of the beam as well as attenuation of the light traveling the distance from the beam to the sensors. In various embodiments, the light produced by the interaction of the probe with the particle field may include scattering, fluorescence, or both. Also, in preferred embodiments, the property-specific concentrations can be characterized in a plane of interest, a portion that is thin with respect to the particle field. The sensors can be located within a plane passing through and parallel to the plane of interest. Accordingly, large volumes of the particle field need not be sampled in addition to the plane of interest in order to determine the accurate particle-specific distributions within the plane of interest. Further embodiments enable the user to determine the distributions of a spray or aerosol by traversing or sweeping a narrow laser beam, such as a gaussian beam or a beam having a circularly symmetric cross-section, i.e., largely cylindrical in shape, through the portion of the ensemble of particles to be characterized. Minimizing the size of the scanning beam to coincide with the field imaged by the sensors can reduce the effects of secondary emission. In addition, in various embodiments of the invention, distributions of properties of particle fields including non-spherical particles can be obtained.
Another aspect of the invention comprises a method of determining a property distribution in a collection of particles. In this method, a beam of excitation light is directed into the collection or ensemble of particles and propagated along a specific optical path within the collection of particles. Scattering and/or fluorescence such as laser induced fluorescence (LIF) from particles along the optical path is produced by the excitation light, the particles additionally scattering and otherwise diminishing or extinguishing the excitation light propagating along said optical path. Measurements of the amount of excitation light entering and exiting the collection of particles are performed and compared to determine the extent to which the excitation light is extinguished by the collection of particles. Scattering and/or fluorescence signals originating from various points along the optical path is determined with detectors thereby detecting the respective scattering and/or fluorescence signals, the scattered and/or fluorescence signals undergoing additional scattering and absorption as they propagate away from the optical path and towards the detectors used to detect the signals. The property distribution of the particles is computed along the optical path using the scattering and/or fluorescence signals, while accounting for the extinction of the excitation light propagating along the optical path, and while accounting for attenuation of the scattered and/or fluorescence signals.